1. Field of the Invention
The present invention is directed to a method for verifying evoked response in the atrium, as well as to an apparatus for conducting the method.
2. Description of the Prior Art
The term xe2x80x9cevoked responsexe2x80x9d means the electrical activation which takes place in the myocardium by a pacing pulse which is emitted by a pacemaker. The presence or occurrence of an evoked response following a pacing pulse means that the pulse was successful for its intended purpose, i.e., artificially stimulating a heartbeat. The absence of an evoked response following a pacing pulse means that one or more parameters of the pacing pulse, such as its energy content or its timing relationship to other pulses, must be adjusted. A chronic absence of evoked response can be indicative of a more serious problem, such as a failure of the electrode lead system or a dislodged electrode lead.
Verifying evoked response in the atrium following a pacing pulse presents a difficult problem. Many techniques are known for verifying capture, which is the successful depolarization and contraction of a cardiac chamber caused by a stimulation pulse from a pacemaker. One-to-one capture occurs when each pacemaker output pulse results in a contraction of the appropriate chamber. A conventional autocapture system verifies capture on a beat-by-beat basis. It would be advantageous to be able to verify evoked response in the same or a similar manner so that an evoked response system could be used in conjunction with a conventional autocapture system.
It is an object of the present invention to provide a method and an apparatus for verifying evoked response on a beat-by-beat basis.
It is also an object of the present invention to provide a method and apparatus for verifying evoked response which can be used in conjunction with a conventional autocapture system.
The above objects are achieved in accordance with the principles of the present invention in a method and an apparatus for verifying evoked response wherein a conventional bipolar pacing electrode is placed in the right atrium, and impedance in the right atrium is measured between the tip and the ring of the electrode. The measured impedance is processed within a time window after emission of a stimulation pulse, and evoked response is verified if the processed impedance signal satisfies a predetermined criterion within the time window. The predetermined criterion can be a comparison of the processed impedance signal, or a characteristic thereof, with a threshold. Alternatively, the predetermined criterion can be comparison of the morphology (i.e. signal curve shape) of the processed impedance signal with a stored morphology (template). In an embodiment of the invention, the time window for making the impedance measurement is determined in dependence on the time that the stimulation pulse to the right atrium is emitted, in order to save battery current. The frequency range of the measured impedance is preferably approximately 50 Hz. The measured impedance signal can be high pass filtered, to obtain the typical delta Z signal that is used for capture verification. Particularly if the predetermined criterion is a morphology comparison, a typical bandwidth for the impedance signal can be between 2 Hz and 40 Hz, with the detection window starting 40 ms after the stimulation pulse and ending at 160 ms after the stimulation pulse. In another embodiment, the bandwidth can be between 4 Hz and 40 Hz and the time window can start 80 ms after the stimulation pulse and end at 180 ms after the stimulation pulse.
In an embodiment, the sensing amplifier, and other components in the impedance signal processor, can be enabled by the pacing logic to operate in the aforementioned time window, and this enabling can be varied dependent on the pacing rate. Additionally or alternatively, the high pass filter can be operated by the pacing logic to select a bandwidth for processing the impedance signal, and this selection also can be varied dependent on the pacing rate.
The simplest algorithm for determining whether the predetermined criterion has been satisfied is to use the amplitude of the processed impedance signal and to compare this amplitude to a simple amplitude threshold, with the processed impedance signal being set to zero at the start of the time window. In order to suppress noise, the impedance signal can be integrated within the time window. An evoked response is then verified if the processed signal reaches a predetermined threshold.
As noted above, another alternative is to employ a morphology comparison. For this purpose, in an embodiment, a morphology comparator can have data representing a curve stored therein, with the curve of the incoming impedance signal being compared to the stored curve. In a further embodiment, a number of different curves (templates) can be stored in a temp late memory, to which the morphology comparator has access. One of the stored curves can be selected by the pacing logic, dependent on pacing rate and/or other pacing conditions, with the selected curve end being supplied to the morphology comparator for comparison with the processed impedance signal.